1. Field of the Invention
The present invention relates to a solid polymer electrolyte, a solid polymer fuel cell and a method for manufacturing the solid polymer fuel cell. More specifically, the present invention relates to a solid polymer fuel cell suitable as a power source for automobiles, a small-scale stationary generator, a cogeneration system or the like, a method for manufacturing the solid polymer fuel cell, and a solid polymer electrolyte used in the solid polymer fuel cell.
2. Description of the Related Art
Solid polymer fuel cells include a membrane-electrode assembly (MEA) as a basic unit in which electrodes are bonded to both surfaces of a solid polymer electrolyte membrane. General electrodes for use in solid polymer fuel cells have a bilayer structure consisting of a diffusion layer and a catalyst layer. The diffusion layer is a layer for supplying a reactive gas and electrons to the catalyst layer, and is made of a carbon paper, a carbon cloth, etc. The catalyst layer is a part where an electrode reaction takes place, and is commonly composed of a composite of carbon supporting an electrode catalyst, e.g., platinum, and a solid polymer electrolyte.
As the electrolyte membrane constituting the MEA or the electrolyte in the catalyst layer constituting the MEA, a highly oxidation resistant perfluorinated electrolyte (i.e. an electrolyte including no C—H bond in the polymer chains) is generally used, for example, Nafion (a registered trademark for products manufactured by DuPont), Aciplex (a registered trademark for products manufactured by Asahi Kasei Corporation, Japan), Flemion (a registered trademark for products manufactured by ASAHI GLASS CO., LTD., Japan), etc.
General perfluorinated electrolytes are highly oxidation resistant, but are very expensive. For the purpose of manufacturing solid polymer fuel cells at low costs, the use of hydrocarbon electrolytes (i.e. electrolytes including C—H bonds but no C—F bond in their polymer chains) and partially fluorinated hydrocarbon electrolytes (i.e. electrolytes including both C—H bonds and C—F bonds in their polymer chains) is under consideration.
However, there remain some unsolved problems for commercializing solid polymer fuel cells as power sources for automobiles, etc. For example, hydrocarbon electrolytes are cheaper than perfluorinated electrolytes, but have a disadvantage in that they tend to be deteriorated by peroxide radicals. On the other hand, perfluorinated electrolytes are highly oxidation resistant when compared to hydrocarbon electrolytes. However, perfluorinated electrolytes have problems that they are gradually deteriorated by peroxide radicals under extreme conditions such as the fuel cell. As a result, F− (fluoride ions) are dissolved therefrom (see, “Non-patent Publication 1”).
Various proposals have been made to solve the above problems.
For example, Patent Publication 1 discloses a proton-conducting polymer membrane in which hydrogen atoms of sulfonic acid groups contained in a sulfonated polyphenylenesulfide membrane are partially replaced by Mg, Ca, Al or La. Patent Publication 1 also describes that the partial replacement of hydrogen atoms of sulfonic acid groups with a metal, e.g., Mg, causes a deterioration in the proton conductivity of the membrane but improves the oxidation resistance of the membrane.
[Patent Publication 1] Japanese Patent Unexamined Publication No. 2004-018573
[Non-patent Publication 1] D. E. Curtin et al., Journal of Power Science 131 (2004) 41-48
However, to attain high oxidation resistance of the proton-conducting polymer membrane disclosed in Patent Publication 1, it is necessary to add large amounts of metal ions. As a result, the electrical conductivity of the membrane drops considerably. On the other hand, when small amounts of metal ions are added to increase the electrical conductivity of the membrane, high durability of the membrane cannot be achieved.
Hydrocarbon electrolytes include C—H bonds in their polymer chains. Although protons of proton-conducting substituents contained in hydrocarbon electrolytes are partially replaced by metal ions to protect the proton-conducting substituents, the other parts of the electrolytes are inevitably deteriorated by peroxide radicals. For this reason, although the proton-conducting polymer electrolyte membrane disclosed in Patent Publication 1 is practically applied to solid polymer fuel cells, sufficiently high durability of the solid polymer fuel cells is not attainable.